MDEV-23672: Partly revert an incorrect fix

In commit 7eda556 we removed a
valid debug assertion.

AddressSanitizer would trip when writing undo log for an INSERT
operation that follows the problematic ALTER TABLE because the
v_indexes would refer to an index object that was freed during
the execution of ALTER TABLE.

The operation to remove NOT NULL attribute from a column should
lead into any affected indexes to be dropped and re-created.
But, the SQL layer set the ha_alter_info->handler_flags to
HA_INPLACE_ADD_UNIQUE_INDEX_NO_WRITE|ALTER_COLUMN_NULLABLE
(that is, InnoDB was asked to add an index and change the column,
but not to drop the index that depended on the column).

Let us restore the debug assertion that catches this problem
outside AddressSanitizer, and 'defuse' the offending ALTER TABLE
statement in the test until something has been fixed in the SQL layer.

mysql-test/suite/innodb/r/instant_alter_bugs.result

@@ -422,11 +422,9 @@ col_text text not null,
 col_int_g integer generated always as (col_int) unique,
 col_text_g text generated always as (substr(col_text,1,499)) )
 engine innodb row_format = redundant;
-insert into t1 values (0, 'a', default, default);
-insert into t1 values (null, 'b', default, default);
+insert into t1 (col_int,col_text) values (0, 'a'), (null, 'b');
 alter table t1 modify column col_text text null, algorithm = instant;
-insert into t1 values (1, null, default, default);
-insert into t1 values (null, null, default, default);
+insert into t1 (col_int,col_text) values (1, null), (null, null);
 update t1 set col_text= 'd';
 select * from t1;
 col_int	col_text	col_int_g	col_text_g

mysql-test/suite/innodb/t/instant_alter_bugs.test

@@ -445,11 +445,13 @@ create table t1 (
     col_int_g integer generated always as (col_int) unique,
     col_text_g text generated always as (substr(col_text,1,499)) )
 engine innodb row_format = redundant;
-insert into t1 values (0, 'a', default, default);
-insert into t1 values (null, 'b', default, default);
+insert into t1 (col_int,col_text) values (0, 'a'), (null, 'b');
+# FIXME: remove the following to trigger the bug
+--disable_query_log
+alter table t1 modify column col_text text null, force;
+--enable_query_log
 alter table t1 modify column col_text text null, algorithm = instant;
-insert into t1 values (1, null, default, default);
-insert into t1 values (null, null, default, default);
+insert into t1 (col_int,col_text) values (1, null), (null, null);
 update t1 set col_text= 'd';
 select * from t1;
 check table t1;

storage/innobase/handler/handler0alter.cc

@@ -605,6 +605,7 @@ inline bool dict_table_t::instant_column(const dict_table_t& table,
 
 	for (unsigned i = 0; i < n_v_def; i++) {
 		dict_v_col_t& v = v_cols[i];
+		DBUG_ASSERT(v.v_indexes.empty());
 		v.base_col = static_cast<dict_col_t**>(
 			mem_heap_dup(heap, v.base_col,
 				     v.num_base * sizeof *v.base_col));